This invention relates to image displays such as television cathode ray picture tubes, and is concerned specifically with an improved method for aluminizing image display faceplates.
The luminescing material, commonly a layer of one or more phosphors of different color emissions deposited on the inner surface of an image display faceplate, is usually "aluminized." The aluminizing process comprises the depositing of an electron-pervious film of aluminum on the phosphors. The film increases the brightness of the display by acting as a mirror to reflect toward the viewer the light produced by the phosphors when activated. The film typically also carries a high-voltage charge to act as an electron-attractive ultor electrode for the display. The thickness of the film is typically about 2,000 Angstroms.
For maximum display brightness and brightness uniformity, it is essential that the aluminum film be as smooth and as mirror-like as possible, and devoid of blemishes such as holes or blisters. Other necessary qualities include firm adherence of the film to the phosphor layer, and uniform thickness of the film for uniform electron penetration.
A problem arises in achieving these qualities primarily because of the unsmooth characteristics of the phosphor layer. As is well-known in the art, the problem is largely resolved by the depositing of a film of an organic material such as a lacquer on the phosphor layer. The film acts to fill in uneven areas of the phosphor layer, providing a smooth surface upon which the aluminum film can be deposited and take on the smooth characteristic of the organic film. It is common practice to remove the organic film by baking the tube following the aluminizing step.
It has proved difficult in manufacture to attain firm adherence of aluminum films so deposited. Physical and thermal shock during manufacture and when transporting, can detach appreciable sections of an inadherent film. A drop of water falling on the surface of the film during the manufacturing process can also detach a section of the film. The consequent difference in brightness of such non-aluminized areas is highly perceptible to the viewer. As a result, the image display, whether a cathode ray picture tube or an image display panel, must be rejected. In addition to poor adherence, highly visible holes and blisters can develop in the aluminum film as a result of the presence of dirt on the phosphor layer before the organic film is deposited.
To promote firm adherence of the phosphor to the glass of the viewing screen, it is a common, well-known practice to "prewet," or "precoat" the phosphor-bearing screen prior to the application of the phosphor with an aqueous slurry which includes a predetermined percentage of a silica compound and insoluble polyvinyl alcohol. To promote attachment of the aluminum film to the phosphor, it is also a known practice to "rewet" the deposited phosphor with an adherence-promoting solution commonly comprising a solution of which a silicate is the main constituent. Rewetting solutions are typically highly alkaline, with a pH value in the range of 10 to 13. The high alkalinity is attributable to the siliceous component commonly used. Such alkaline solutions have proved to be less than satisfactory in promoting adherence of the aluminum film.
Wilcox, in U.S. Pat. No. 3,940,508 discloses a precoat layer consisting of a highly insoluble, low-molecular-weight polyvinyl alcohol which is thinly dispersed and dried upon the inner surface of a television picture tube faceplate panel. The precoat is applied in the form of an aqueous slurry in which the polyvinyl alcohol is rendered highly insoluble by adjusting the pH value of the slurry to a value of less than about 3. The precoat, when uniformly dispersed and dried, is alleged to provide a highly adherent layer on the glass faceplate to which the phosphor is in turn said to be highly adherent.
The problem of poor adherence of the aluminum film is resolved according to the present invention by means of an improved method for rewetting the phosphor layer prior to the application of the organic film and the aluminizing film.